1. Field of the Invention
The present invention relates to an air classifier which can produce toner particles having a desired particle size.
2. Description of the Prior Art
In general, toner particles are produced through the steps of: mixing raw materials, melting and kneading the mixed materials, cooling the kneaded materials, and pulverizing the cooled materials, classifying the pulverized materials. Recently, toner particles are required to have small particle size and narrow particle-size distribution, as copying techniques for color images and digital images require very fine copy images.
In order to produce such toner particles as above mentioned, the classifying process needs particular classifying mechanism with high precision and high throughput capacity. The kneaded materials are conventionally classified by the following described pulverizing and classifying system, as shown in FIG. 4.
Roughly pulverized materials are introduced from an introduction port 1 after kneading. A blower 2 sucks air in pipeline to generate air-flow as shown by arrows. The roughly pulverized materials ride on the air flow, and are transferred to be put into an air-classifier main body 3. Toner particles larger than desired particle size are transferred to pulverizer 4, toner particles having desired particle size being transferred to cyclone 5, and super fine particles being transferred to bag-filter 6. As initially roughly-pulverized materials have a large particle size, almost all of the particles are transferred to the pulverizer 4. The toner particles finely pulverized in the pulverizer 4 are transferred again to the classifier main body 3 and subjected to the above mentioned classification. Toner particles as having a large particle size are repeatedly pulverized and classified until desired particle size is achieved. Then the toner particles having desired particle size are transferred to the cyclone 5 and collected. Super fine particles are transferred to the bag filter 6, although some toner particles transferred to the cyclone 5 contain super fine particles not removed in the classifier 3.
Toner particles collected in the cyclone 5 are accumulated on the upper part of double dampers 7. While the downside valve is closed, the upside valve is opened, so that toner particles fallen onto the downside valve. After the upside valve is closed, the down side valve is opened. Thereby, while the conditions of air flow inside the pipelines are maintained, toner particles can be taken out of the inside. Double dampers 8 are also arranged at the lower portion of the bag filter. Super fines collected in the bag filter 6 can be taken out of the inside while the air-flow conditions inside pipe lines are maintained.
An air-classifier, often used in the above system, may be exemplified by Dispersion Separator (DS type; made by Nippon Pneumatic MFG K.K.) utilizing swirling air, the sectional view of which is shown in FIG. 5. The reference number 21 shows a casing body. The reference number 22 shows a downside casing connected to a lower portion of the casing body 21, playing a role of a hopper 23 at the same time. A classifying area 24 is formed between the casing body 21 and the downside casing 22. A dispersion room 25 is formed at the upper port of the casing body 21. A raw material-supplying pipe 26, through which a mixture of primary air flow with pulverized materials are supplied, is connected to the peripheral upper port of the dispersion room. A conical center core 27 with high center portion is arranged at the lower port inside the dispersion room. Ring-shaped supplying grooves 28 are arranged at the lower peripheral edge portion of the center core 27. An exhaustion pipe 31 for fine particles is arranged at the bottom center of the classifying area. A conical separator core 29 with high center portion is arranged on the top of exhaustion pipe 31. Ring-shaped exhaustion grooves 30 for roughly pulverized particles are formed on the peripheral lower portion of the separator core 29. Secondary air flow inlets 32 and 33 for supplying secondary air flow are arranged on the lower peripheral wall of the classifying area. The secondary air flow disperses powder materials and accelerates swirling speed. The reference number 34 shows an air exhaustion pipe for introducing super fines to the bag filter (reference number 6 in FIG. 4). The reference number 35 shows an outlet for exhausting roughly pulverized particles to the pulverizer (reference number 4 in FIG. 4).
The characteristic of such a classifier as above mentioned is that the difference between centrifugal force and centripetal force working on the pulverized particles is utilized when the secondary air flow makes the pulverized particles revolve semi-freely in the classifying area, so that classifying conditions can be adjusted depending on particle size of particles to be classified. Throughput capacity is almost fixed by the capacity of the dispersion room and the classifying area or by the total flow of the first air flow and the secondary air flow.
However, there arises problems such as lowering of classifying efficiency the finer the toner particles, the higher is the adhering force of fine particles to toner particles in such an above mentioned pulverizing and classifying system of air-flow type. In particular, this problem is remarkable when an organic boron compound is used as a charge controlling agent. The cause of the problem is thought as follows. As the adhering force of fine particles to toner particles increases, dispersion of the fine particles becomes difficult. In particular, cohesion of particles becomes strong when organic boron compounds are used, resulting in difficulty of dispersion of fine particles. As a result, as aggregates go into the classifying area, in other words, as the aggregates can not be broken sufficiently in the dispersion room, they are classified as they are. Therefore, toner particles with fine particles adhered thereto are classified as they are although the toner particles themselves have a proper particles size. The toner particles with fine particles adhered thereto are pulverized again together with large toner particles to be over-pulverized, and unnecessary fine particles increase, resulting in lowering of classifying efficiency.
Further, the incorporation of fine particles into a toner product is not ignored as the adhesion of fine particles to toner particles increases. The use of such toner product causes problems such as filming and fog.
In order to solve such problems as described above, Japanese Patent Laid-Open No. Hei 7-80415 discloses that more than two throttles or convexes are arranged in the direction towards pipe center from inner wall of raw material-supplying pipe so that aggregates of toner particles may be pulverized. As such, a classifier causes pressure loss, it is thus required to increase a capability of an air-flow generator above that necessary, resulting in a problem of effective throughput capacity.